Per-pixel detector bias control

ABSTRACT

A pixel includes a photo-diode, an integration capacitor arranged to receive a photo current from the photo-diode and to store charge developed from the photo current; and an injection transistor disposed between the photo-diode and the integration capacitor that controls flow of the photo current from the photo-diode to the integration capacitor, the injection transistor having a gate, a source electrically coupled to the photo-diode at a first node, and a drain electrically coupled to the integration capacitor. The injection transistor is a silicon-oxide-nitride-oxide-silicon (SONOS) FET having its gate set to a SONOS gate voltage to control a detector bias voltage of the photo-diode at the first node.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of an earlier filing date from U.S.Provisional Application Ser. No. 62/721,695 filed Aug. 23, 2018, theentire disclosure of which is incorporated herein by reference.

BACKGROUND

The present disclosure relates to a digital pixel imager and, inparticular, a digital pixel circuit that includes a per-pixel biascontrol.

In legacy analog imagers, particularly infrared imagers, photo-currentfrom a detector diode is integrated by a well or integration capacitorcoupled to the detector diode, and then once per video frame, thevoltage or charge of the well capacitor is transferred to a down-streamanalog-to-digital converter (ADC), where the voltage is converted to abinary value.

One type of in-pixel ADC circuit utilizes a direct injection (DI)transistor. In such a circuit charge from a photo-diode is accumulatedover an integration capacitor. Charge is accumulated, in general, untila readout time. When that time is reached, the charge stored inintegration capacitor is provided to a readout circuit. Such circuitscan either integrate then read or read while integrating circuits.

Control of the flow of current from the photo-diode is controlled by aninjection transistor. The gate of the injection transistor is coupled toa bias voltage. The level of this voltage can be selected by the skilledartisan and is used, in part, to keep the photo-diode in reverse bias.

However, in some cases detector bias non-uniformity affects the overallperformance and yield (pixels not meeting performance) of an imagingarray.

SUMMARY

According to one embodiment, a pixel includes a photo-diode, anintegration capacitor arranged to receive a photo current from thephoto-diode and to store charge developed from the photo current; and aninjection transistor disposed between the photo-diode and theintegration capacitor that controls flow of the photo current from thephoto-diode to the integration capacitor, the injection transistorhaving a gate, a source electrically coupled to the photo-diode at afirst node, and a drain electrically coupled to the integrationcapacitor. The injection transistor is asilicon-oxide-nitride-oxide-silicon (SONOS) FET having its gate set to aSONOS gate voltage to control a detector bias voltage of the photo-diodeat the first node.

According to another embodiment, a pixel that includes a photo-diode; anintegration capacitor arranged to receive a current from the photo-diodeand to store charge developed from the current; and asilicon-oxide-nitride-oxide-silicon (SONOS) field-effect transistor(FET) disposed between the photodiode and the integration capacitor isdisclosed. The SONOS FET is configured to control a detector biasvoltage of the photo-diode

In a pixel of any prior embodiment, the detector bias voltage at thefirst node is equal to: V_(DETBIAS)=V_(dd)−(V_(TSONOS)−V_(GS)), whereV_(GS) is the gate to source voltage of the injection transistor,V_(TSONOS) is the SONOS gate voltage, and V_(dd) is a voltage applied ona side of the photo-diode opposite the first node.

In one embodiment, in a pixel of any prior embodiment, the gate isformed of a gate stack that includes a layer of silicon nitride thatstores current to set the SONOS gate voltage.

In one embodiment, in a pixel of any prior embodiment, the layer ofsilicon nitride is formed of Si₃N₄ or Si₉N₁₀.

In one embodiment, a pixel of any prior embodiment, further includes areset switch coupled in parallel with the integration capacitor.

In one embodiment, in a pixel of any prior embodiment, the SONOS FETcomprises a gate, a source electrically coupled to the photodiode at afirst node, and a drain electrically coupled to the integrationcapacitor, wherein the gate is set to a SONOS gate voltage to controlthe detector bias voltage of the photodiode at the first node.

In one embodiment a method of operating a pixel is disclosed. The methodincludes coupling a photo-diode to a source of an injection transistor,wherein the injection transistor is asilicon-oxide-nitride-oxide-silicon (SONOS) FET; coupling an integrationcapacitor to a drain of the injection transistor such that theintegration capacitor can receive a photo current from the photo-diodeand store charge developed from the photo current; and setting a SONOSgate voltage on the injection transistor to control a detector biasvoltage of the photo-diode at a first node disposed between thephoto-diode and the integration capacitor.

According to a method of any prior embodiment, the detector bias voltageat the first node is equal to: V_(DETBIAS)=V_(dd)−(V_(TSONOS)−V_(GS)),where V_(GS) is the gate to source voltage of the injection transistor,V_(TSONOS) is the SONOS gate voltage, and V_(dd) is a voltage applied ona side of the photo-diode opposite the first node.

According to a method of any prior embodiment, the gate is formed of agate stack that includes a layer of silicon nitride that stores currentto set the SONOS gate voltage.

According to a method of any prior embodiment, the layer of siliconnitride is formed of Si₃N₄ or Si₉N₁₀.

According to a method of any prior embodiment, setting the SONOS gatevoltage includes providing a pulse to the gate of the injectiontransistor, wherein the length of pulse is proportional to the level ofthe SONOS gate voltage.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts:

FIG. 1 is a schematic diagram illustrating image detector in accordancewith embodiments;

FIG. 2 is a schematic diagram illustrating a unit cell of FIG. 1 thatincludes a SONOS injection transistor; and

FIG. 3 is graph showing an example of pulse width applied to set thegate bias of the injection transistor.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram illustrating an image detector 100 inaccordance with embodiments. Such a detector 100 may be deployed, forexample, on a satellite or other airborne apparatus such as an aircraftor any land- or sea-based tactical application in which it is arequirement that frame rate not be limited by array size. Image detector100 may be a focal plane array (FPA), active pixel sensor (APS) or anyother suitable energy wavelength sensing device. The image detector 100may be used as a component of a photographic and/or image capturingdevice, such as a digital camera, video camera or other similar device.The image detector 100 may include a detection device 120 and ananalog-to-digital converter (ADC) 140.

The detection device 120 includes an array of photosensitive/energywavelength sensitive detector unit cells 160 arranged in an X×Y matrix.Each of the detector unit cells 160 may accumulate charge or produce acurrent and/or voltage in response to light incident upon the detectorunit cell 160 and may correspond to a pixel in a captured electronicimage. One or more of the detector unit cells 160 may include aphotovoltaic detector (e.g., a photovoltaic single absorber detector ora photovoltaic multi-absorber (multi-junction) detector), a barrierdevice detector, a position sensitive detector (PSD) or other suitabledetector.

The ADC 140 may be used for processing of the incident light (e.g., tocreate an image representative of the incident light). For example, theADC 140 interfaces with the detection device 120 to receive a signal,such as the accumulated charge or the current and/or voltage produced inresponse to light incident upon the detector unit cells 160. In oneembodiment, the ADC 140 includes a read out integrated circuit (ROIC)200 that accumulates voltage/current and produces a digital output.

The ADC 140 may include an array of ADC unit cells that are arranged inan X×Y matrix corresponding to the orientation of the X×Y matrix of thedetector unit cells 160. Thus, each ADC unit cell may be interconnectedwith a corresponding detector unit cell 160 by way of one or more directbond interconnects, such as direct metal-to-metal interconnects orindium interconnects. Each detector unit cell can also be referred to aspixel herein.

In such systems and as generally described above, it is known to utilizean injection transistor to place a diode in the detector unit cell intoreverse bias so that it will generate current when exposed to light. Insuch a systems a generally universal voltage is applied to the gate ofall of the injection transistors to achieve the reverse bias. However,given variations in both detector and the injections transistors,various cells in a particular array may be unusable when such universalgate bias voltage is applied. For example, in some cases, a particulardiode may produce too much so-called “dark current” when the gate of itsassociated injection transistor is coupled to the universal gate biasvoltage. The dark current may be so great that it makes any measurementfrom the diode unusable and, as such, makes the particular diodeunusable. Similar problems may also exist when due to signal droopcauses the voltage applied to a gate of a particular injectiontransistor results in a lower than desired bias voltage being applied tothe diode. Still further, variations in the injection transistorsthemselves can have negative effects. Herein is disclosed a system whereeach pixel includes an injection transistor in form of asilicon-oxide-nitride-oxide-silicon (SONOS) FET that can be individuallyprogrammed so that a particular diode can operate in a useable manner.This can alleviate problems due to variations in a universal gatevoltages across an array and address the variations in individual diodesor transistors or both.

FIG. 2 shows a pixel 160 according to one embodiment and includes aphoto-diode 110. The pixel 160 includes an input node 114, an SONOSinjection transistor 112, an integration capacitor 115, and a resetswitch 130. Charge from the photo-diode 110 is accumulated over theintegration capacitor 115. Charge is accumulated, in general, until areadout time. When that time is reached, the charge stored inintegration capacitor 115 is provided to ADC 140. Then, the capacitor115 can be reset by closing the reset switch 130 upon receipt of a resetsignal 125.

In one embodiment, the ADC 140 is connected to the pixel 160 to create aso-called “digital pixel.” In such a case, the ADC 140 can include acomparator 120 that compares the charge on the capacitor 115 to athreshold voltage (Vref). When the voltage across the integrationcapacitor 115 (referred to as Vint herein) exceeds Vref, the pixel 160is reset via the reset switch 130 that receives the reset signal 125.During a reset, a voltage equal to the difference between Vref andVreset is subtracted from the integration capacitor 115. Of course, thisis but one example and not meant as limiting. That is, the disclosureherein can be applied to digital and non-digital pixels.

In the case of a digital pixel, each reset event is accumulated(counted) with a digital counter circuit 135. At each frame, a“snapshot” of the contents of the digital counter circuit 135 is copiedinto a register or memory and read out, line by line. This digitalcounter circuit 135 operates to exponentially increase the well capacityQ_(INT) of the integration capacitor 115 by a factor of 2^(N), where Nis the size of the digital counter circuit 135. Thus, by conserving thephoto-charge relationship within a frame period, this type of pixel 160may achieve improved signal-to-noise ratio.

After the integration time expires, any residual charge accumulated onthe integration capacitor 115 can be read out by, for example, a singleslope ADC or any other type of ADC (not shown). Of course, when thepixel is not a digital pixel, rather than counting times a comparatorexceeds a threshold, all accumulated charge in a frame is read out bythe ADC.

As in prior systems, control of the flow of current from the photo-diode110 is controlled by the SONOS injection transistor 112. Similar to theprior art, the gate of the SONOS injection transistor 112 is coupled toa bias voltage (Vbias). The level of this voltage is selected, in part,to keep the photo-diode 110 in reverse bias. In such a reverse biasconfiguration, the photo-diode 110 will produce current for two reasons.The first is so called dark current and the other is due to light beingabsorbed by the diode 110. As discussed above, the range of reverse biasfor a diode to perform satisfactorily can vary from diode to diode andcan be quite narrow in some instances and different voltages may need tobe applied to the gate of SONOS transistors for the diode to operate ina manner that it can produce usable results.

In prior systems an nFET or a pFE was typically used as the injectiontransistor. In such systems, the gate of the injection transistor wastypically connected to an injection transistor voltage (V_(DI)) and aglobal V_(DI) was applied to to the gates of each direct injectiontransistor in the array. Accordingly, in a typical pixel, the detectorbias voltage can be controlled by the gate voltage (V_(DI)) as follow:

V _(DETBIAS) =V _(dd)−(V _(DI) −V _(GS));

where V_(GS) is the gate to source voltage of the FET.

However, detector non-uniformity (sensor response, noise, leakage) aswell as non-uniformity of the global bias circuitry that supplies V_(DI)(transistor matching, V_(DI) supply voltage droop, etc.) across thearray can affect the overall performance and yield (pixels not meetingperformance) of the imaging array.

Of course, there are ways to apply non-uniformity correction (NUC) tothe output of the pixel but that require significant processing powerand introduce latency in image formation. Some attempts to removedetector and circuitry non-uniformity. For example, an N bitdigital-to-analog converter implemented at the pixel level has beenproposed but can take significant space in the unit cell layout. Anotherapproach is to sample V_(bias) on a capacitor but this can increasecircuit noise.

Herein, to remove the effects the above described non-uniformities, thetypical injection transistor can be replaced with the illustrated SONOS(silicon-oxide-nitride-oxide-silicon) FET 112. The SONOS FET 112includes a gate 200, a source 202 and a drain 204. The SONOS FET 112includes a layer of silicon nitride 206 disposed therein in the gatestack. The layer of silicon nitride 206 can be one of: Si₃N₄ or Si₉N₁₀.The SONOS FET for each photodiode can be set individually to account forvariations in the performance of each photodiode and the SONOS FET 112itself.

The source 202 of the SONOS FET 112 is electrically connected to thephotodiode 110 at node 114 and the drain 204 is electrically connectedto the integration capacitor 115. As shown, the connections at thesource 202 and drain 204 are direct connections but this is not requiredunless specifically recited.

In operation, a voltage can be stored on the gate 200 of the SONOS FET112. This stored voltage can set the bias of the diode of discussed morefully below. In more detail, one or more of the gates of the SONOS FETs112 across an array can each have a bias in the gate 200 that is held inthe silicon nitride layer 206. Once stored, this value can heldindefinitely and serves to raise the voltage at which the SONOS FET 112will conduct. That is, the stored voltage will keep node 114 at levelthat allows for a particular diode to produce a useable output. As such,the bias circuitry non-uniformities as well as the diode and injectiontransistor non-uniformities mentioned above can be removed as each cellcan be programmed to the desired bias voltage level. The overall imagingperformance and yield can be improved.

In more detail, the voltage stored on the gate 200 of the SONOS FET 112can be referred to herein as V_(TSONOS). Accordingly, the detector biasvoltage V_(DETBIAS) at node 114 can be represented by:

V _(DETBIAS) =V _(dd)−(V _(TSONOS) −V _(GS)).

It shall be understood by the skilled artisan that V_(GS) will be uniquefor each FET. As shown by trace 302 in FIG. 3, when a pulse is appliedto the gate 200, the duration of the pulse will set the level of theV_(TSONOS). Similarly, trace 304 shows an example of V_(TSONOS) beingreduced. This can be utilized to vary the level of V_(TSONOS) of aparticular cell. The value of V_(TSONOS) can be reduced such that itsfollow trace 304 by application of a negative pulses to the gate of theSONOS transistor 112. In another embodiment, if a new value ofV_(TSONOS) is desired, a constant negative bias can be applied to thegate of the the SONOS transistor 112 to erase any prior programming. Anew value can then be programmed in the manner described above.

The SONOS FET can store V_(TSONOS) because the gate of the SONOS FET 112includes a layer of silicon nitride 206. In more detail, in operation,when gate 200 is biased positively, electrons from the transistor source202 and drain 204 regions tunnel get trapped in the silicon nitridelayer 206. This results in an energy barrier between the drain 204 andthe source 202, raising the threshold voltage V_(TSONOS) (thegate-source voltage) necessary for current to flow through the FET 112.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiments were chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

While the preferred embodiments to the invention have been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A pixel comprising: a photo-diode; an integrationcapacitor arranged to receive a photo current from the photo-diode andto store charge developed from the photo current; and an injectiontransistor disposed between the photo-diode and the integrationcapacitor that controls flow of the photo current from the photo-diodeto the integration capacitor, the injection transistor having a gate, asource electrically coupled to the photo-diode at a first node, and adrain electrically coupled to the integration capacitor; wherein theinjection transistor is a silicon-oxide-nitride-oxide-silicon (SONOS)FET, and wherein the gate is set to a SONOS gate voltage to control adetector bias voltage of the photo-diode at the first node.
 2. The pixelof claim 1, wherein the detector bias voltage at the first node is equalto:V _(DETBIAS) =V _(dd)−(V _(TSONOS) −V _(GS)); where V_(GS) is the gateto source voltage of the injection transistor, V_(TSONOS) is the SONOSgate voltage, and V_(dd) is a voltage applied on a side of thephoto-diode opposite the first node.
 3. The pixel of claim 1, whereinthe gate is formed of a gate stack that includes a layer of siliconnitride that stores current to set the SONOS gate voltage.
 4. The pixelof claim 3, wherein the layer of silicon nitride is formed of Si₃N₄ orSi₉N₁₀.
 5. The pixel of claim 1, further including a reset switchcoupled in parallel with the integration capacitor.
 6. A method ofoperating a pixel, the method comprising: coupling a photo-diode to asource of an injection transistor, wherein the injection transistor is asilicon-oxide-nitride-oxide-silicon (SONOS) FET; coupling an integrationcapacitor to a drain of the injection transistor such that theintegration capacitor can receive a photo current from the photo-diodeand store charge developed from the photo current; and setting a SONOSgate voltage on the injection transistor to control a detector biasvoltage of the photo-diode at a first node disposed between thephoto-diode and the integration capacitor.
 7. The method of claim 6,wherein the detector bias voltage at the first node is equal to:V _(DETBIAS) =V _(dd)−(V _(TSONOS) −V _(GS)); where V_(GS) is the gateto source voltage of the injection transistor, V_(TSONOS) is the SONOSgate voltage, and V_(dd) is a voltage applied on a side of thephoto-diode opposite the first node.
 8. The method of claim 6, whereinthe gate is formed of a gate stack that includes a layer of siliconnitride that stores current to set the SONOS gate voltage.
 9. The methodof claim 8, wherein the layer of silicon nitride is formed of Si₃N₄ orSi₉N₁₀.
 10. The method of claim 6, wherein setting the SONOS gatevoltage includes providing a pulse to the gate of the injectiontransistor, wherein the length of pulse is proportional to the level ofthe SONOS gate voltage.
 11. A pixel comprising: a photo-diode; anintegration capacitor arranged to receive a current from the photo-diodeand to store charge developed from the current; and asilicon-oxide-nitride-oxide-silicon (SONOS) field-effect transistor(FET) disposed between the photodiode and the integration capacitor,wherein the SONOS FET is configured to control a detector bias voltageof the photo-diode.
 12. The pixel of claim 11, wherein the SONOS FETcomprises a gate, a source electrically coupled to the photodiode at afirst node, and a drain electrically coupled to the integrationcapacitor, wherein the gate is set to a SONOS gate voltage to controlthe detector bias voltage of the photo-diode at the first node.
 13. Thepixel of claim 12, wherein the detector bias voltage at the first nodeis equal to:V _(DETBIAS) =V _(dd)−(V _(TSONOS) −V _(GS)); where V_(GS) is the gateto source voltage of the injection transistor, V_(TSONOS) is the SONOSgate voltage, and V_(dd) is a voltage applied on a side of thephoto-diode opposite the first node.
 14. The pixel of claim 12, whereinthe gate is formed of a gate stack that includes a layer of siliconnitride that stores current to set the SONOS gate voltage.
 15. The pixelof claim 14, wherein the layer of silicon nitride is formed of Si₃N₄ orSi₉N₁₀.
 16. The pixel of claim 11, further including a reset switchcoupled in parallel with the integration capacitor.